1. Field of the Invention
The present invention relates to an ashing method and, more particularly, an ashing method of ashing resist by use of oxygen.
2. Description of the Prior Art
At present, miniaturization of a semiconductor integrated circuit is rapidly being progressed and also a semiconductor wafer used for forming the semiconductor integrated circuit is being increased in size. A lithography technique is indispensable for fabricating the semiconductor integrated circuit. In general, a mask which is formed by coating, exposing, and developing resist is employed in the lithography technique. In addition to the lithography technique, the mask made of the resist may be employed to implant the impurity into predetermined areas of a semiconductor layer.
Since the resist is removed in a final stage of the lithography step or the ion-implantation step, a technique which can mate with the miniaturization and the high integration degree of the semiconductor integrated circuits is needed as a resist removing method. For example, improvement in ashing uniformity, improvement in high ashing selectivity with respect to a thin film as an etching object, prevention of damage of the substrate, high cleanliness, high throughput, etc. are requested.
For this reason, techniques which are able to reduce damages of the substrate and the film, reduce residues and particles after ashing, and increase throughput of the ashing process are needed in ashing.
Meanwhile, if an N well and a P well are formed in one silicon substrate, p-type impurity and n-type impurity are separately implanted while covering respective parts of the silicon substrate with the resist mask. In the step of forming MOS transistors in the N well and the P well respectively, the p-type impurity and the n-type impurity are ion-implanted selectively into the N well and the P well while covering one of the N well and the P well with the resist mask since different impurities are to be implanted into the N wells and the P wells respectively. There are phosphorus, arsenic, etc. as the n-type impurity, for example, and there are boron, etc. as the p-type impurity, for example.
In the event that a dosage of the p-type impurity or the n-type impurity is enhanced in such ion-implantation, a deteriorated layer is formed on a surface of the resist film because of introduced impurity. Because such deteriorated layer is difficult to be removed by the ordinary method, a special method must be employed. Generation of the deteriorated layer has been set forth in articles, for example, Shuzo FUJIMURA et al., JAPAN JOURNAL OF APPLIED PHYSICS, VOL. 28, NO. 10, October 1989, pp. 2130-2136, and K. Shinagawa et al., DPS 1992, pp.75-80. The deteriorated layer is generated when a dosage is increased higher than about 5.times.10.sup.15 atoms/cm.sup.2.
As the ashing method, there are a wet ashing method and a dry ashing method. The wet ashing is preferable in that there is no damage to the substrate and the film, but a chemicals bath is contaminated every ashing process and the particles introduced into the chemicals bath are adhered again to the substrate and the film.
As the dry ashing method, there is a method of exposing directly the resist to plasma by use of the ashing equipment shown in FIG. 1, for example.
FIG. 1 shows a dry etching equipment having a configuration in which the deteriorated layer of the resist is directly exposed to plasma. In FIG. 1, a gas supply pipe 2 is provided on an upper portion of a chamber 1 of the ashing equipment. A gas shower plate 3 having plenty of holes is then fitted to an lower end of the gas supply pipe 2 to be directed to a substrate stage 4. A high frequency power supply 5 is then connected to the substrate stage 4. Exhaust ports 6 are then provided on a bottom portion of the chamber 1.
In order to execute the ashing of the resist, a pressure in an interior of the chamber 1 is reduced, then a high frequency voltage is applied to the substrate stage 4, and then the substrate W on which the resist having the deteriorated layer thereon is formed is placed on the substrate stage 4. When an oxygen mixed gas is supplied into the chamber via the gas supply pipe 2 and the gas shower plate 3, plasma such as oxygen plasma is generated between the gas shower plate 3 and the substrate stage 4. As a result, the deteriorated layer of the resist is exposed to the oxygen plasma and then removed.
Not only the resist and the deteriorated layer but also the substrate W and the film formed on the substrate W are exposed to the plasma in the ashing process. Therefore, there has been such a problem that the substrate W and the film are damaged by ions in the plasma.
On the contrary, as disclosed in Patent Application Publication (KOKAI) 4-286317, Patent Application Publication (KOKAI) 8-288260, and Patent Application Publication (KOKAI) 8-69896, there are methods in which the deteriorated layer of the resist is removed by ion and the undeteriorated layer is ashed by the radical. However, since these techniques employ ions in removing the deteriorated layer, merely such an advantage can be expected that an influence of damage onto the substrate and the film caused by ions is reduced.
As the dry ashing method, a downflow ashing method in which ashing is carried out with the use of radical of the reaction gas including reduced ions has been set forth in Patent Application Publication (KOKAI) 7-86146.
According to the ashing method using the radical in the prior art, if a power of the microwave to generate the plasma is increased in order to enhance a resist removing rate (ashing rate), a part of the deteriorated layer has remained on the substrate and therefore the deteriorated layer has not been able to be removed completely.
According to the downflow ashing method in the prior art, if the substrate temperature is increased more than 150.degree. C. in order to enhance the ashing rate, resist burst is generated to thus contaminate the chamber. The resist remained in the chamber causes generation of the particles and also causes the residue of resist on the substrate. In general, such resist burst is called a "pumping phenomenon" and has a bad influence upon the throughput.